In semiconductor devices, usually a high purity coating material is applied for preventing the surface of a semiconductor chip particularly PN junction and bonding pad portion, as well as fine aluminum or gold wires from undergoing deterioration of characteristics or corrosion caused by moisture or impurities and also for shielding a thermal and mechanical stress relaxation memory element from .alpha.-ray. And a sealing by molding with a synthetic resin or a hermetic seal using a metallic or ceramic vessel is applied for fixing and enclosing the surface-coated semiconductor chip, bonding wires and lead wires for external connection.
Materials used for protecting the surface of a semiconductor chip are generally called junction coating resins or undercoating materials. As these materials there are used specially purified silicone resins and polyimide resins.
Junction coating resins are classified into a solution type with resin dissolved in a solvent and a solventless type not containing a solvent, and generally assume a liquid state.
As packaging forms, there are a one-pack type and a two-pack type. As resin curing mechanisms, there are a condensation type which involves the formation of a by-product and an addition type which does not produce a by-product. As curing methods, there are a room temperature curing method, a heat curing method and a radiation curing method. As cured resin forms, there are gel-like form which is superior in stress relaxing effect and adhesion, a rubbery form which is superior in stress relaxing effect, and a hard resin form which is superior in mechanical strength and dielectric strength.
Thus, there are a great variety of junction coating resins, but in their semiconductor use, the most suitable resin is selected according to the semiconductor's surface state, design withstand-voltage, shape and packaging method. The selected junction coating resin is applied onto the surface of a semiconductor chip so as to afford desired area and coating thickness usually by the use of a dispenser. Thereafter, the junction coating resin is cured by a method suitable for the resin, for example, by curing in a high temperature and high humidity atmosphere, by curing in a high temperature atmosphere, or by irradiation of an ultraviolet ray having a suitable wave length.
After application and curing of the junction coating resin, there generally is performed a sealing with a molding resin such as, for example, a silicone resin or an epoxy resin, or a hermetic seal using a metallic or ceramic vessel for the purpose of fixing and enclosing the semiconductor chip, bonding wires and lead wires for external connection.
However, in semiconductor devices using such conventional liquid junction coating resins as described above, there are involved various problems, for example:
.circle.1 It is difficult to coat only a predetermined portion exactly with a junction coating resin; in other words, even a portion not to be coated is often coated with the resin. Further, since the other portion is contaminated by a component by-produced during heat curing of the junction coating resin, an attempt to connect bonding wire or perform soldering results in a defective connection or a poor adhesion of a sealing resin.
.circle.2 If a wire bonding type resin-sealed IC obtained by coating the surface of a semiconductor chip and 25.mu.-dia. gold bonding wires with a liquid addition reaction type junction coating silicone resin, then curing the resin into a rubbery state by heating and thereafter sealing the circumference by molding with an epoxy resin, is subjected to a heat shock test in which the IC is held at -50.degree. C. for 30 minutes and immediately thereafter is held at 150.degree. C. for 30 minutes, there occurs breaking of the bonding wires from around 20 cycles which is presumed ascribable to expansion and contraction of the junction coating silicone resin in contact with the bonding wires.
.circle.3 If a wire bonding type resin-sealed transistor obtained by coating the surface of a semiconductor chip and 30.mu.-dia. aluminum bonding wires with a liquid addition type junction coating silicone resin, then curing the resin into a rubbery state by heating and subsequently sealing the circumference by molding with a silicone resin, is subjected, after application thereto of a silicone compound for heat dissipation, to a heat fatigue test in which turning on and off of power is repeated, there occurs breaking of the bonding wires from 2,000 cycles which is presumed ascribable to swelling of the junction coating resin in contact with the bonding wires caused by the silicone oil which has permeated the resin.
.circle.4 As to a hybrid IC, if a condensation type junction coating silicone resin of a high viscosity is applied to a very small portion on a substrate and cured into a rubbery state by allowing it to stand at room temperature for 24 hours and then gold wires are connected to bonding pad portion adjacent to the applied portion, there frequently occurs a defective connection of the bonding wires which is presumed ascribable to the diffusion of the liquid silicone resin to the pad portion.
It is an object of the present invention to prevent the occurrence of the above-mentioned problems.
It is another object of the present invention to provide a semiconductor device having a high reliability.
It is a further object of the present invention to provide a semiconductor device free from breaking of bonding wires, cracking of solder and peeling of beam leads.
It is still a further object of the present invention to provide a semiconductor device not causing a defective bonding and superior in adhesion of a sealing resin.